In the conventional way, the master cylinder is filled with brake fluid and equipped with a main hydraulic piston intended to receive an actuating force composed of an input force and of a boost force both acting in an axial direction.
Furthermore, the pneumatic booster is capable of being operated by applying the input force to an operating rod controlling the opening of a valve in order to exert the actuating force on the main hydraulic piston of the master cylinder, the booster including a rigid casing divided in leaktight fashion into two chambers by means of a moving partition capable of being acted upon by a difference in pressure between the two chambers resulting from the opening of the valve and of driving along a pneumatic piston which can move with respect to the casing and carries the valve, the input force being transmitted via a reaction disc on which the pneumatic piston also presses in order to provide it with at least some of the boost force.
A device of this type is well known in the prior art and is described, for example, in document U.S. Pat. No. 4,491,058.
These braking devices have, by way of advantage, and as a result of the use of a pneumatic piston which can move with respect to the rigid casing, the fact that the total travel available for the operating rod and therefore for the brake pedal, is relatively long, and this constitutes a condition which needs to be satisfied in order to ensure optimum control of the deceleration of the vehicle under braking.
However, in cases of sharp braking intended to relieve an emergency situation, these braking devices have the major drawback that in order to obtain a relatively high pressure in the brake circuit, they need a brake pedal travel which is substantially longer than that required to obtain the same pressure in the circuit during braking under normal conditions when the emergency nature of the braking is less of an issue.
Given the urgency of the braking, the driver is not always aware of this lengthened travel of the brake pedal, because he presses down on it with rapidly varying substantial force in order to obtain the braking effect required by this emergency situation.
Braking systems have therefore been developed which include various sensors which are sensitive to the force applied to the brake pedal and/or to the rate of application of this force so as to correct, using a pneumatic solenoid valve operated by a microprocessor receiving the signals from these sensors, the pressure difference prevailing in the booster in order to make it increase more rapidly so as also to increase the boost force more rapidly and therefore correct the lengthening of the response time of the booster by increasing the boost force.
However, although the sensors and the microprocessor have very short response times, the mechanical components on which the sensors are fitted have relatively high inertia or response time which means that the electronic correction system does not come into play until a relatively long time after the emergency situation has been detected.
In parallel, boosted braking devices have been developed in which the reaction exerted on the operating rod is no longer provided mechanically by a reaction disc but by the hydraulic pressure prevailing in the master cylinder.
In these devices, the main hydraulic piston of the master cylinder itself includes a hollow moving cylinder communicating with the master cylinder and receiving at least some of the boost force, and inside which there slides in leaktight fashion and in the axial direction a secondary hydraulic piston capable of receiving at least the input force, elastic means exerting an elastic force between the secondary hydraulic piston and the moving cylinder and urging the secondary hydraulic piston in the direction of the master cylinder, at least one opening being made in the moving cylinder so as to make the interior of the latter communicate with the interior of the master cylinder.
Such a device is described, for example, in document FR-A-2,658,466.
These devices with hydraulic reaction have as their main advantage the fact that however intense the braking action and however great the rate of application of the input force, their characteristic operating curve, namely the curve giving the pressure in the master cylinder as a function of the intensity of the input force of the booster, is unchanged. These devices therefore show no lengthening in the response time in the case of emergency braking.
The fact of equipping these hydraulic reaction devices with sensors for detecting emergency situations and with a pneumatic solenoid valve for increasing the pressure in the rear chamber of the booster thus yields a clear advantage by comparison with devices having mechanical reaction, because it plays a part in improving the normal operating curve of the booster, rather than seeking to correct a degraded operating curve.